As a stutterer, I can assure you, the best thing you can do is to just be patient. Let us finish our sentences. Let the person know that you listen to them and understand what they say. When talking in person, always try to keep eye contact. Also, in my opinion, if you feel comfortable with it, there is no harm in asking the person how they want you to behave, specifically in finishing their sentences etc. But to sum up: let the person finish their sentence, no matter how much they are struggling to do so. Make the person understand that you have their fullest attention.